Over 1 million hip and spine fractures each year are associated with osteoporosis, a disease of low bone mass that contributes to increased morbidity, mortality and economic strain on our medical system. Effective low-cost prevention strategies such as bone-loading exercise, could lower the incidence of osteoporotic fractures without an increase in medical costs, and provide an alternative to drug therapy. Activity associated with high magnitude forces such as fast running and jumping, have been shown to increase hip bone mass by 1.2%-4% in premenopausal women which may translate into a 20-30% reduction in hip fracture risk. In addition, high impact exercise may also produce benefits that are maintained long-term. However, a specific exercise prescription for improving bone health has not been determined. To date, no single study has examined the interactive effects of jump magnitude (height) and jump number (repetitions) on bone mineral density by systematically varying the height and number of jumps performed. In addition, no study has evaluated the effects of loading exercise on multiple measures of bone health, in order to quantify the effects of exercise on bone strength, apart from bone mineral density. Determining the optimal dose of jump exercise for improving bone strength will allow us to determine a specific exercise prescription for bone health in premenopausal women and will be useful in future projects that intend to employ jump training to target bone health. The long-term objective of this line of research is to determine how impact loading improves bone quality to ultimately reduce fracture risk. We propose a randomized, controlled, trial to compare the effects of a 9-month supervised exercise program using three levels of load magnitude (4", 8", 12" jump height) and three load repetitions (10, 50, 100 jumps per session), on three dimensions of bone health (bone density, remodeling and strength) in 300 premenopausal women aged 18-42y. Bone density is the most widely recognized dimension of bone health and clinically accepted index of fracture risk. Bone remodeling reflects the dynamic state of bone and can predict fracture risk independent of bone density. Bone strength, represented by Femur Strength Index, is a reflection of the geometry and structural competence of bone. Low-cost osteoporosis prevention strategies including jumping exercises, could lower the incidence of osteoporotic fractures without an increase in medical costs, and provide an alternative to drug therapy. This project will allow us to determine the minimum effective dose of jumping exercise required to benefit bone health in premenopausal women and will lead to future research on how exercise improves bone quality and reduces fracture risk.